This invention relates to evaporative cooling towers used in conjunction with electric power generation plants.
Many industrial processes use surface water from rivers and lakes for cooling purposes. Usually the water is returned to the same source but at a temperature substantially higher, sometimes as much as 30.degree. F higher, depending upon climatic conditions. Such a substantial change in water temperature may adversely affect the aquatic ecology and this effect is commonly referred to as "thermal pollution."
In the United States, the greatest single source of waste heat discharged into surface waters is electrical power generation. The amount of water withdrawn for this purpose alone in the United States is estimated to be about 40 trillion gallons annually. With an increasing demand for electrical power, there is rising concern about the thermal pollution resulting from such increased generation. The Federal Water Pollution Control Administration of the Department of the Interior has held extensive hearings and the potential ecological hazard to aquatic life caused by the dissipation of waste heat through natural water flow has been described in detail. See, for example, Thermal Pollution and Aquatic Life, Scientific American, March 1969, Volume 220, Number 3.
Various approaches have been suggested to decrease the thermal pollution problem. U.S. Pat. No. 3,760,868 discloses a system which incorporates a municipal domestic water distribution system as a heat sink to lessen the heat load injected into the external water supply. U.S. Pat. No. 3,851,495 discloses a system which incorporates an underground water storage and heat dissipation space formed by a nuclear explosion as a heat sink to lessen thermal pollution. Numerous other approaches have been proposed which involve decreasing the amount of heat injected into the external water supply by either utilizing alternative liquid heat sinks, or by transferring the heat load into the atmosphere.
Cooling towers of both the mechanical and natural draft type have been utilized for some time to exchange the heat load from water into the atmosphere. The water that is circulated through the cooling towers is continually changing because some water is lost to the atmosphere by evaporation and "drift," as described below. Also, it is necessary to continually withdraw a portion of water, termed blowdown water, to avoid a buildup of scale in the circulating water system. Makeup water is injected into the tower to make up the losses from the sources described above. In the past, the blowdown water has been exhausted directly into the external water supply. The temperature of this blowdown water in relation to the temperature of the external water supply is a function of climatic conditions, load of the tower, etc . . . , but in some instances it can be as much as 30.degree. F higher than the external water supply. This difference in temperature could have a significant ecological impact.